A HIGH - PRESSURE PROCESS FOR THE PRODUCTION OF MULTICRYSTALLINE MELAMINE AND MULTICRYSTALLINE MELAMINE POWDER

Abstract

This invention relates to a high-pressure process for the production of multicrystalline melamine in which solid melamine is obtained by a melamine melt being transferred to a vessel where the melamine melt is cooled with an evaporating cooling medium, wherein the melamine melt, having a pressure between 15 MPa and 25 MPa and a temperature between the melting point of melamine and 450°C, is sprayed via spraying means and cooled with an evaporating cooling medium within a vessel in an ammonia environment at an ammonia pressure of 0.1 -25 MPa, the melamine melt being converted into melamine powder having a temperature below 150°C, the melamine powder then being cooled to a temperature below 50°C, the powder being set in motion mechanically over at least part of the cooling range and being cooled directly or indirectly, and the ammonia pressure being released at a temperature below 270°C. The invention further relates to processes for the use of a multicrystalline melamine powder having the following characteristics: specific surface area: 0.7-5 m2/g colour APHA less than 17 melamine: > 98.5 wt% melam:< 1.3 wt%. This invention also includes multicrsystalline melamine prepared by the above process.

Full Text

contrast, contains a substantial amount of crystals having a crystal size greater than 50 µm. On the SEM pictures the crystallographic crystal faces (large relatively flat areas) in the case of melamine crystallized from water are clearly discernible. These structures can be seen on Figures 1 and 2; Figure 1 comprises SEM pictures (Figure lA: 50x and Figure IB: 1500x) of particles having a so-called cauliflower structure, whereas Figure 2 comprises SEM pictures of melamine crystallized from water (Figure 2A: 50x and Figure 2B: 500x). The photographs of the products were produced using a Philips SEM 515 at an accelerating voltage of 15 kV.
The applicant has now also found that melamine can be produced with continuously high purity by the melamine melt which comes from the melamine reactor and has a temperature between the melting point of melamine and 450°C first being treated with gaseous ammonia (0.1-15 mol of ammonia per mole of melamine) and then being sprayed via spraying means and cooled by means of an evaporating cooling medium within a vessel in an ammonia envirormient at an ammonia pressure of 0.1-25 MPa, the melamine melt being converted into melamine powder having a temperature between 200°C and the solidification point of melamine, the melamine powder then being cooled to a temperature below 50°C, other cooling methods also being used if required. If required, the powder can be further cooled in the same vessel or in another vessel by the powder being set in motion mechanically and being cooled directly or indirectly.
Melamine powder has poor flow and fluidization characteristics and a low temperature equalization coefficient (poor thermal conductivity). Standard cooling methods such as a fluidized bed or a packed moving bed can therefore not readily be implemented on a commercial scale. The applicant has found, however, that the colour of the melamine powder, in particular, is adversely affected if the melamine remains at a high temperature for too long. Effective control of the dwell time at high

temperature has therefore proved critical. It is therefore important to be able to cool the melamine powder effectively.
Surprisingly, it proved possible to cool melamine powder rapidly, despite the poor flow characteristics, by setting it in motion mechanically and at the same time cooling it directly or indirectly. Indirect cooling means that the mechanically agitated bed of melamine powder is brought into contact with a cooling surface. Direct cooling means that the mechanically fluidized bed is brought into contact with a cooling medium, for example ammonia or an air stream. A combination of direct and indirect cooling is obviously also possible.
Accordingly, the present invention provides a high-pressure process for the production of multicrystalline melamine wherein solid melamine obtained by a melamine melt is transferred to a vessel where the melamine melt is cooled with an evaporating cooling medium, characterized in that the melamine melt, having a pressure between 15 MPa and 25 MPa and a temperature between the melting point of melamine and 450°C, is sprayed via spraying means and cooled with an evaporating cooling medium within a vessel in an ammonia environment at an ammonia pressure of 0.1-25 MPa, the melamine melt being converted into melamine powder having a temperature below 150°C, the melamine powder then being cooled to a temperature below 50°C, the powder being set in motion mechanically over at least part of the cooling range and being cooled directly or indirectly, and the ammonia pressure being released at a temperature below 270°C.
Accordingly, the present invention also provides multicrystalline melamine powder having the following properties: specific surface area: 0.7-5 m2/g; level of oxygen containing component 98.5 wt%; melam:

WE CLAIM:
1. A high-pressure process for the production of multicrystalline melamine wherein solid melamine obtained by a melamine melt is transferred to a vessel where the melamine melt is cooled with an evaporating cooling medium, characterized in that the melamine melt, having a pressure between 15 MPa and 25 MPa and a temperature between the melting point of melamine and 450°C, is sprayed via spraying means and cooled with an evaporating cooling mediimi within a vessel in an ammonia environment at an ammonia pressure of 0.1-25 MPa, the melamine melt being converted into melamine powder havings a temperature below 150°C, the melamine powder then being cooled to a temperature below 50°C, the powder being set in motion mechanically over at least part of the cooling range and being cooled directly or indirectly, and the ammonia pressure being released at a temperature below 270°C.
2. The process as claimed in claim 1, wherein the melamine melt is treated with 0.1-15 mol of ammonia per mole of melamine.
3. The process as claimed in claim 1, wherein the melamine melt is cooled by at least 5°C before spraying into the vessel.
4. The process as claimed in claim 1, wherein ammonia is introduced into the melamine melt in such a way that a gas/liquid mixture is sprayed via the spraying means.
5. The process as claimed in claim 1, wherein the evaporating cooling medium is liquid ammonia.

6. The process as claimed in claim 1, wherein the melamine melt is sprayed at an outflow velocity greater than 20 m/s.
7. The process as claimed in claim 1, wherein the powder remains in contact with ammonia at a pressure of 0.1 - 25 MPa over a period of 1 min-5 hours, with the options of the product remaining at virtually the same temperature during the said contact time or being cooled down.
8. The process as claimed in claim 1, wherein the melamine melt is sprayed via spraying means within a vessel in an ammonia environment at a pressure of 1-11 MPa.
9. The process as claimed in claim I, wherein the powder obtained by spraying is further cooled by means of an apparatus provided with means for moving powder mechanically and provided with means for cooling powder directly or indirectly.
10. The process as claimed in claim 6, wherein the means for moving powder mechanically comprise a rotating screw, drum, bowl, discs, disc segments or pipes.
11. The process as claimed in claim 6, wherein the apparatus has a cooling area of 50-5000 m2.
12. The process as claimed in claim 1, wherein the powder is cooled by at least 35°C by the powder being set in motion mechanically and being cooled diretly or indirectly.

13. A process for the preparation of a melamine-formaldehyde resing, wherein the
melamine used is a multicrystalline powder that has the following properties:
specific surface area: 0.7-5 m2/g
colour APHA less than 17 melamine: > 98.5 wt% melam:
14. A process for the dissolution of melamine in a solvent wherein said melamine
is a multicrystalline powder that has the following properties:
specific surface area: 0.7-5 m^/g colour APHA less than 17 melamine: > 98.5 wt% melam:
15. The process as claimed in claim 15, wherein the solvent is water or a water/formaldehyde mixture.
16. Multicrystalline melamine powder having the following properties specific surface area: 0.7-5 m2/g
level of oxygen containing component 98.5 wt% melam:
17. The laminates or coatings made from the resin as claimed in claim 13.
18. Multicrystalline melamine powder substantially as herein described with reference to the accompanying drawings.